(i) Field of the Invention
The present invention relates to a device for exciting a gas, of the surfaguide type, in which the gas is excited by a surface wave plasma, in particular an atmospheric-pressure surface wave plasma.
The invention also relates to an apparatus for treating a gas incorporating such an excitation device.
Another effective exciting device for this application is known by the name "surfatron-guide".
(ii) Description of Related Art
One particularly advantageous application example of these types of devices is the plasma treatment of a chemically non-reactive gas containing impurities consisting of perfluorinated greenhouse-effect gaseous compounds or of volatile organic compounds.
To do this, the gas to be treated and the impurities which it contains are placed in an electric field which is intense enough to produce an electrical discharge by ionizing the gas molecules, this discharge being caused by stripping off electrons from the initially neutral gas molecules.
Under the action of the discharge, the molecules of the gas are dissociated in order to form radicals of smaller sizes than the initial molecules and, consequently, when appropriate, individual atoms, these atoms or fragments of molecules thus excited not appreciably giving rise to any chemical reaction.
Thus, after passing through the discharge, the gas atoms or molecules become de-excited and recombine respectively, before becoming intact again on leaving the discharge.
In contrast, the impurities undergo, by excitation, irreversible dissociation and irreversible transformation by forming new molecular fragments having chemical properties different from those of the initial molecules, which are consequently capable of being extracted from the gas by an appropriate subsequent treatment.
A surfatron-guide has a hollow structure made of an electrically conductive material, having a first end closed off by a moveable waveguide plunger forming a short-circuit and a second part which extends perpendicularly to the first part and in which is coaxially mounted a tube made of a dielectric material, through which tube the gas to be treated flows.
The second part is provided with a tuning plunger which can move axially in order to adapt the impedance of the device.
This type of electromagnetic field applicator is satisfactory for creating a surface wave plasma at atmospheric pressure.
However, it has a certain number of drawbacks, in particular due to its cost, because of the greater complexity of its construction.
However, another type of gas-exciting device is known, this being called a "surfaguide".
This type of excitation device has a hollow structure forming a waveguide, made of electrically conductive material, which is intended to be connected to a microwave generator provided with a passage through which a hollow discharge tube made of a dielectric material is intended to pass, the gas to be excited flowing through the tube, and with a wave-concentrating region designed to concentrate the microwave radiation produced by the generator onto the tube, during operation of the device, for the purpose of producing a surface wave plasma in the gas.
The surfaguide has no tuning plunger and is therefore less expensive than the surfatron-guide. Furthermore, the length of the plasma created by the surfaguide is, for the same power, slightly longer than that of the plasma created by the surfatron-guide.
However, the density of the plasma column produced by the surfatron-guide is locally higher than for the surfaguide.
In addition, under certain operating conditions, the surfaguide is less effective than the surfatron-guide when discharge tubes having a diameter greater than 20 mm are used at a frequency of 2.45 Ghz.
Moreover, for high operating powers, radiation losses occur in the environment of the surfaguide, these being highly prejudicial to the energy balance of the device and also causing reliability and safety problems.